Wiki info about Spitfire LF Mk IX

The Supermarine Spitfire was a British single-engine, all-metal, low-wing monoplane fighter. While the Mk VII and Mk VIII variants was being designed, a Spitfire Mk III was tested in September 1941 with the new Rolls Royce 60 in the hopes of developing an interim fighter to use as a stop gap measure. The new Spitfire performed exceptionally well, and was rushed into production as the Mk IX. The Air Fighting Development Unit described the Mk IX as ‘outstandingly better than the Mk V especially at heights above 20,000 feet’ and even though it was not fitted with the modified control surfaces under development for the Mk VII and Mk VIII, the Mk IX was more than capable of meeting the Focke-Wulf FW190 on favourable terms. The Mk IX was fitted with the Type C ‘universal’ wing and originally carried the tried and tested combination of two 20mm cannon and four 0.303 inch machine guns, but in later versions the four 0.303 machine guns were replaced with two .50 calibre guns.
A total of 5,665 Mk IX were produced and 262 more Mk Vs were converted to Mk IX. Intended as a transitional stop-gap measure for the Mk VII and Mk VIII, the variant remained in production until the end of the war, ultimately becoming the most mass-produced Spitfire version. This mark was also used as a fighter-bomber and as a reconnaissance aircraft. Further modifications were incorporated into the Mk IX throughout its service life, such as a gyroscopic gunsight, bubble canopy and modified engine intercooler. Widely considered to be the most capable fighter in the world at the time of its introduction, it would take the introduction of the Rolls Royce Griffon engine to push the Spitfire into its next stage of evolution.